In the xerographic process as described in U.S. Pat. No. 2,297,691, a base plate of relatively low electrical resistance such as metal, etc., having a photoconductive insulating surface coated thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly in the base plate in proportion to the intensity of light to which any given area is exposed, the charge being substantially retained in non-exposed areas. After exposure, the coating is contacted with electrostatic materials which adhere to the remaining charges to form a powder image corresponding to the latent electrostatic image remaining after exposure. The powder image then can be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be. Since dissipation of the surface electrostatic charge is proportional to the intensity of the impinging radiation, light sources of uniform and sufficient intensity must be provided so that the photoconductive insulator can be properly exposed.
Low pressure metal or metal halide lamps are a near optimum illumination source for photocopiers producing black and white output copies from black and white and multi-colored originals.
With respect to line copy, the optimum goal of any black and white photocopying apparatus is to make the image areas on the copy as black as possible. In other words, one would like a minimum of energy reflected from the image areas of the original while reflecting a maximum from the background region. Obviously, it is impossible to copy all colored backgrounds as white while concurrently copying all colored images as black.
From prior experience, it appears that most colors that are utilized as images on an original tend to be located at the extremes of the visible spectrum, i.e., blues and reds, whereas yellow, for example, is seldom utilized for images. Colored backgrounds are pastel (desaturated) and can usually be considered as tinted white paper which may be explained in part on well known principles of physiological optics (photoptic vision).
It then follows that the optimum light source for photocopying apparatus producing black and white output copies from black and white and multi-colored originals produces yellow light whereby black and reds will copy as black, while concurrently most common colored papers have considerable reflectance in yellow (it should be noted that the use of the yellow exposure lamps obviously necessitates a yellow sensitive photoreceptor). However, the typical prior art photocopying apparatus utilizes aperture fluorescent lamps which generate colored light.
Low pressure sodium lamps represent a commercially available yellow light source. Present commercial sodium lamps, such as those manufactured by N. V. Phillips, have several disadvantages for photocopying applications associated therewith.
The principal problem is that a long warm-up period is required before the lamp may be operated at its optimum light output or efficiency, i.e., at an operating temperature of approximately 260.degree. C which in turn corresponds to a vapor pressure of approximately 0.005 Torr. For example, whereas unassisted fluorescent lamps require only a matter of seconds to reach peak radiance, unassisted sodium lamps require several minutes to heat the lamp and achieve the optimum vapor pressure for radiation output. The prior art has sought to reduce long warm-up times in sodium lamps by maintaining the lamp continuously energized. However, additional problems arise if the sodium lamp is on continuously. For example, the photoreceptor may fatique when it is continuously flooded with light produced by said sodium lamp, the heat generated by the sodium lamp may harm the photoreceptor, and continuous operation of the lamp may add to the cost of a customer's electrical bill. Further, most commercially available low pressure sodium lamps are in a U-tube configuration which causes a fit problem in most photocopying apparatus because of its large diameter, the U-shaped lamp also emitting light in all directions which is inefficient and unacceptable for photocopying use.
U.S. Pat. No. 3,914,649, issued on Oct. 21, 1975, discloses a low pressure sodium vapor lamp for use in photocopying apparatus operated in a pulsed mode. Although advantages are attained in using such a lamp in a pulsed mode, it is desirable in many applications that a low pressure sodium vapor lamp of high efficiency and short warm-up periods be provided without the additional components required for pulsed operation. Copending application of James F. Shaw et al, Ser. No. 668,875 filed concurrently herewith discloses a low pressure sodium vapor lamp of this type.
Another problem in using a low pressure sodium lamp is the control of the sodium vapor pressure in the discharge. If the pressure is too high, resonance trapping occurs and the lamp efficacy drops. If the sodium pressure is too low, the efficacy also drops through for different reasons.
Typically, the sodium pressure required is about 5 .times. 10.sup.-3 Torr, and for a precision load, very tiny amounts of sodium would be required. However, adequate tube lifetimes with precision loaded tubes have not been provided by tube manufacturers because of sodium clean-up as the tube ages.
The problem can be solved by using a large excess of sodium and regulating the sodium pressure by changing the lamp temperature. This requires that the lamp must be held continuously at about 260.degree. C for optimum light output since the lamp temperature cannot change more than a few degrees before the output falls off. This necessitates a precise and dependable temperature control system capable of holding a substantially constant lamp temperature regardless of ambient temperature variations.